X-ray tube



June 7, 1938.

Filed May 29., 1936 3 Sheets-Sheet 1 r- I l l l l l l l l'l l l m 1/ 0149155 kl/rm/v ATTOE/VfK C. V. LITTON June 7, 1938.

X-RAY TUBE Filed May 29, 1936,

, 3 Sheets-Sheet 2 4.5 IIIIIII'III'W l l l f l f l'l'l'l'l'l'l'l' TTOQAIEK c. v. LITT ON X-RAY TUBE June 7, 1938.

r 3 Sheets-Sheet 3 Filed May 29, 1936 y N lHlI w 6 w m 5 z a r 7 :55 N 2/ ll ,HHH 1IF I N J.J W E, A a 6/ sf 3 m p 2 .6 w w 2 focal-spot over the Patented June 7, 1938 UNITED STATES PATENT or'rlcs x-nsr runs Charles V. Litton, San Mateo County, Calif. Application my 29, ms, Serial No. cam

15 Claims. (onto-s5) This invention relates to x-ray tubes and has to do more particularly with tubes designed for use in the making of x-ray pictures.

For radiographic purposes it is desirable always to use a tube with a very small focal spot because by so doing the maximum definition is obtainable. possible to use a very small spot where a considerable thickness must be penetrated because the target would not withstand the great temperature which would result from concentrating the necessarily high milliamperage on such a small area for the time required to efiect a suiiicient exposure. Therefore it is necessary with ordinary tubes to use larger focal-spots on heavy work and to accept as inevitable the concomitant sacrifice of definition.

The hypothetically possible methods of meeting the aforementioned deficiency are:

(1) To discover a satisfactory target metal that will withstand much higher temperatures than any metal now known;

(2) To discover much more efi'icient means than is now known for dissipating the generated heat and:

(3) To eii'ect a continuous movement of the face of the target so that no one portion of the target is exposed to bombardment for a suiiicient length of time to permit of an excessive temperature rise.

The present invention falls within the third mentioned classification.

It has been proposed heretofore to provide rotating or otherwise continuously moving targets and it is believed that such tubes have been developed commercially; but there are inherent and seemingly unavoidable difiiculties standing in the way of commercial production and use of x-ray tubes of that characterwhich dimculties arise out of the fact that there are moving parts inside the tube which cannot be closely fitted in order to gree of precision.

One of the principal objects of the present invention is to devise an x-ray tube of the type wherein the focal spot continuously traverses the face of the target during operation of the tube, but in which the anode, including the target, is fixed-that is, not rotatable or otherwise moveable with respect to the envelope of the tube-the ultimate objective being an x-ray tube of the class referred to which is free from internal memaintain a proper deand moving electrical contactsand which, in adfor the dissipation of lubricated, yet must be ternating the cathode anode heat than one in which the anode is rotatable.

In order to achieve the above-stated objective it is essential that the cathode beam be moveable laterally of its direction of propagation during operation of the tube; and to eifect such a lateral movement of the beam I prefer to make use of what may be termed electrical deflecting means-operating upon the beam either electrostatically or electromagnetically. But I do not consider that the scope of my invention is limited, in all of its aspects, to any particular mode or means by which the lateral movement of the beam is accomplished.

For radiographic p rposes it is usually necessary, or at least preferable, that the focal spot be substantially stationary in space-that is, from the viewpoint of the object being radiographed and, likewise, from the viewpoint of the photographic film or plate.

In one of its principal aspects, my invention is characterized by the fact that the focal spot is caused to traverse (scan) the face of the target (during operation of the tube) while at the same time the tube as a whole is moved more or less bodily in directional opposition to the targettraversing movement of the focal spot-the focal spot being thereby maintained substantially fixed in space.

There are two known electrical methods by which a cathode beam, 1. e: a beam of electrons,

A cathode beam traversing a magnetic field in space, more or less normally to the magnetic lines of force, will be deflected laterally, crosswise of the magnetic lines of force, just as a metallic conductor passing through a magnetic field and carrying a'current (as in an electric motor) will be caused to move transversely of the lines of force; and, if the magnetic field is .al-

beam will be reciprocated laterally.

The two aforementioned methods of deflecting the cathode beam may be employed individually or jointly. That is to say, the beam may be oscillated and thus caused to scan the target in two directions simultaneously by means either of two sets of electrostatic deflectors or two deflecting magnets or by means of one set of electrostatic deflectors and one deflecting magnet. The two scanning movements of the cathode beam are preferably approximately at right angles to each other-one occurring preferably at a fairly high frequency such as sixty cycles per second while the other occurs at a relatively low frequency, say one to flve cycles per second. The low frequency scanning of the target is in synchronism with the oscillatory physical movement of the tube as a whole and is in a plane such that, from the viewpoint of the object being radiographed, the focal-spot is substantially stationary.

To bring about a predetermined lateral deflection of a cathode beam either electrostatically or electromagnetically, the required electrostatic deflecting potential or magnetic fleld intensity, as the case may be, is more or less proportional to the anode potential. By this I mean that the electrical potential or magnetic force required to deflect the cathode beam in an ordinary X-ray tube to any given extent is more or less directly proportional to the voltage across the anodecathode terminals. With a tube operating at,- let us say, one hundred k. v., the necessary deflecting potential would be quite high. Similarly, a magnetic fleld of very considerable strength would be needed to eflect a like deflection of the beam.

It is very desirable to be able to bring about the requisite deflection of the beam without having to resort to excessively high potentials on the electrostatic deflecting electrodes and, similarly, to avoid the necessity for a very powerful magnetic fleld where the deflecting is accomplished mag-' netically. I have discovered a method and means whereby the deflecting of the beam can be brought about with relatively small deflecting potentials or a relatively weak magnetic fleld, as the case may be. This I accomplish through an arrangement which operates to shield the cathode beam from the influence of the high anode potential until after the electrons which constitute the beam have traveled past the point or points in their space path at which they are laterally deflected-the electrons being caused to move at relatively low velocity through the initial portion of their space path (during which period they are laterally deflected) and thereafter at a greatly increased velocity.

The velocity of electrons in a highly evacuated space path is a function of the space potentialwhich in an ordinary x-ray tube is the potential difference across the anode-cathode terminals. If the space potential actually operating upon the electrons emitted by the cathode can be made materially less than the anode-cathode potential difference, during an initial portion of the electron movement from cathode to anode, the velocity of the electrons through such initial portion of the space path becomes correspondingly less.

In the X-ray tubes illustrated in the accompanying drawings and hereinafter described, there is included a shield on which is impressed a suitable intermediate positive potential. The shield has a somewhat constricted opening through which the cathode beam passes to the target without physical obstruction; but, at the same time, the shield operates as a barrier to the 7i electrostatic lines of force emanating from the anode and wholly or to a large extent prevents the electrons of the cathode beam being influenced by the anode until they have passed beyond the point or points where they are laterally deflected. The electrons, instead of being accelerated initially by the anode fleld, are, instead, influenced wholly or mainly by the fleld set up by the shield potential-the shield, itself, being, in a sense, a sort of intermediate anode. At the same time, the constricted portion of the shield having the opening through which the beam passes is, in effect, a virtual cathode-that is, from the viewpoint of the anode.

It is contemplated that the cathode beam may be in the form of a pencil having a more or less circular cross-section as in a conventional round-focus X-ray tube or that it may have a more or less rectangular cross-section as in a conventional line-focus tube. With a cathode beam of the last mentioned form the scanning of the target need be only in one direction, that is laterally of the linear focal-spot. The scanning in that case would be at a low frequency only, isochronously with the reciprocatory movement of the tube.

Referring to the drawings:

Fig. 1 is a side elevational view of an X-ray tube mounted on a support which is adapted to be reciprocated, together with the tube. This view also depicts a driving motor and speed reduction gear together with a low frequencyalternating current generator or magneto which serves to provide alternating deflecting voltage in synchronism with the reciprocatory movement of the tube;

Fig. 2 is an endwise view of the tube and other parts shown in Fig. 1;

Fig. 3 is a circuit diagram of an arrangement for providing low frequency alternating potential from a source of direct current.

Fig. 4 is a view, mostly in longitudinal section, showing the interior construction of the X-ray tube and also illustrating diagrammatically the electrical connections;

Fig. 5 is a cross-sectional view of the tube taken along the line 5-5 of Fig. 4;

Fig. 6 is a cross-sectional view taken along the line 66 of Fig. 4;

Fig. 7 is a face view of the target showing the focal-spot and illustrating the movement of the focal-spot back and forth across the face of the target;

Fig. 8 is also a view of the face of the target with the path of the focal-spot depicted thereon. This view illustrates the result of a pencillike cathode beam scanning the target in one direction (vertically) at a relatively high frequency, say sixty cycles per second, and in the other direction at a relatively low frequency, say from one to flve cycles per second;

Fig 9 is an external elevational view of an X-ray tube which is much the same as those illustrated in the preceding figures but differing therefrom in that an external deflecting magnet is employed to take the place of one of the two sets of electrostatic deflector plates; 0

Fig. 10 is an elevational view of the deflecting magnet shown in Fig. 8 and a cross-sectional view through the tube illustrating more especially the space relation between the pole pieces of the magnet and the electrostatic deflector plates in- '1 side the tube;

Fig. 11 is a view of the cathode end ternative tube structure in which the beam is of rectangular cross-section, say. a line-focus beam; and

of an alprojected that is to the line l2-l2 of Fig.

Fig. 12 is a cross-sectional view taken along In Figures 1 and 2, reference numeral I designates, as an entirety, an X-ray tube in accordance with this invention comprising an evacuated glass envelope of cylindrical form in which are enclosed all the usual elements including the anode with its target and a thermionic cathode together with other parts shownin Fig. 4. At the anode end of the tube is an air-cooling radiator 2 of conventional form. The left hand or cathode end of the tube is provided with a conventional screw plug 3 which is shown inserted in a screw socket 4. Heating current is supplied to the cathode of the tube through the screw plug 3 and socket 4. The whole tube together with the radiator 2 and socket 4 are mounted on and supported by a pair of uprights 5 and 6, respectively, which, in turn, are mounted on and carried by a cross member I of insulating material. Cross member 1 is provided with an opening 3 through which X-rays are emitted. To the lower side of cross member I and at either end is attached a pair of channel-shaped members 9 and- HJ respectively. These latter are seated in guides II and I2 in which they are free to slide reciprocably as indicated by the double headed arrow in Fig. 2.

Guides II and I2 are secured to and carried on the top of a table or base plate l3, of insulatin material, which would generally form a part of or be carried by the adjustable arm of an X-ray machine.

Mounted upon base plate I3 is an'electric motor l4 having a speed reduction gear enclosed within a housing i 5. A drive-shaft projecting from the speed reduction gear housing has an ofi-set crank pin it, to which is journaled a connecting rod I], one end of which is pivotally connected at l8 to the cross member 'I. With the motor in operation the cross member I together with the X-ray tube and other parts carried by the cross member are reciprocated slow1y-the length of the stroke being madeto equal the distance spanned by the cathode beam in its travel across the face of the target in the same direction as that in which the tube is reciprocated. The scanning of the target and the physical reciprocatory movement of the tube are isochronized so that from the viewpoint of the object being X-rayed the focal-spot remains stationary. As a matter of fact the focal-spot is stationary in space but there ment between the focal-spot and the surface of the target.

Also shown mounted on the table I3 is an elec-- tric generator or magneto l9. This is driven through the speed reduction gear l5 and it is designed to produce an alternating voltage of the same frequency as that at which the X-ray tube is physically oscillated. The voltage from this generator is isochronous with the oscillatory movement of the tube .and is of a magnitude such as to be suitable for application to the low frequency electrostatic deflector plates which will be described later.

As an alternative to erator l9 there may be the low frequency genprovided an arrangement such as shown in Fig. 3 which comprises a direct current source 33 across the terminals of which is connected a potentiometer 34 having two sliding contacts 35 and 36. By moving these contacts continuously, in opposite directions, at the desired low frequency and isochronously with the oscillatory movement of the X-ray tube, the

is a relative movedesired low frequency alternating potential can be produced across the terminals 31 and 38.

Figs. 4, 5 and 6 illustrate somewhat in detail an x-ray tube wherein the cathode beam is in the form of a pencil of or less circular cross-section. The cathode beam is reciprocated at a fairly high frequency, say sixty cycles per second, in directions toward and away from the object to be radiographed-thus producing, in effect, a linear focai-spot-and at the same time the cathode beam is reciprocated slowly at right angles to the previously mentioned plane of reciprocation-thus, in effect, causing the linear focal-spot to move laterally back and forth across the face of the target. Fig. 7 illustrates, approximately, how the focal-spot would appear to an observer and depicts the oscillatory transverse movement of the target. Fig. 8 illustrates the path of the focal-spot when reciprocating in one direction at say sixty cycles per second and in the other direction at say one to five cycles per second.

Referring more particuarly to Fig. 4, the X- ray tube which is designated as a whole by reference numeral i comprises a glass envelope 20 of generally cylindrical form, the interior of which is highly evacuated. Inside the envelope are shown an anode 2i which is usually of copper, a thermionic cathode 22, a "gun or accelerator 23, a pair of electrostatic deflector plates 24 and 25 and a second pair of similar electrostatic deflector plates 26 and 21. Inserted in the anode 2| is a circular tungsten button is provided with a upon it a suitable predetermined positive potential. The function of the gun is to form the electronic discharge into a concentrated beam and propel the beam toward the target. The use of such guns in oscillograph machines is well known.

The electrostatic deflectors 24, 25, 26 and 21 consist each of a small metal plate which may be rectangular in form as shown. Across the deflector plates 24 and 25 is applied suitable magnitude. This low frequency potential is the one which may be supplied by the generator I 9 or the arrangement of Fig. 3. It must be isochronous with the reciprocatory movement of the tube in order that the focalspot may remain stationary in space.

The cylindrical shield 29 has a constricted neck portion at the end thereof adjacent the target, which neck portion defines an opening through which the cathode beam must pass to reach the Shield 29 operates as a barrier to the electrostatic lines of force emanating from the anode, wherefore the electrons forming the cathsmall diameter and more 1 ode beam are not influenced by the target until they have entered and at least partially passed through the opening in the shield-at which point the electrons are quickly accelerated by the high anode potential and thus caused to strike the target at a suitable velocity for the production of X-rays. A positive potential of some suitable intermediate value is impressed upon shield 29 and, as a consequence, the shield performs the function of an anode in that it imparts a propulsive force to the electrons, causing them to move toward the target-but at a lower velocity than that at which they would be propelled if under the influence of the high anode potential. Thus the electrons are caused to move at a relatively low velocity through that part of their space path in which they are acted upon by the deflector plates and, resultantly, a relatively low potential on those plates will answer the purpose. Were it not for the shield 29, the deflecting, potentials would necessarily be quite large in X-ray tubes operating at high anode voltages-a condition which it is highly desirable to avoid. What has been said with respect to the effect of the shield upon the deflecting potentials applies also to its effect upon the requirements as to the intensity of the magnetic fleld which may be employed to deflect the beam as a substitute for electrostatic deflection.

The function of the focusing coil 30 is to concentrate or reconcentrate the cathode beam. It is well known that the effect of a magnetic field upon a cathode beam passing axially therethrough is to concentrate the beam. Thus, if the beam shows any tendency to spread out this may be cured by means of a focusing coil Ill. The focusing coil should not be confused with the magnetic deflecting coil which presently will be described. They perform entirely different functions.

In addition to physical structure, Fig. 4 includes a diagrammatic showing of electrical connections. There is depicted a transformer 39 having a primary winding 40, a secondary winding 4i, which supplies heating current to the cathode and a secondary winding 42 which supplies sixty cycle potential to the electrostatic deflector plates 24 and 25. The low frequency generator 19 shown in Fig. 1 is also illustrated diagrammatically in Fig. 4. A source of direct current 43 supplies biasing potential to the focusing shield ll; another direct current source 44 supplies a positive potential to the gun 23 and the cylindrical shield 29 has impressed upon it a still higher positive potential by .virtue of the inclusion in its circuit of the additional direct current source 45. The maximum positive potential is of course applied to the anode 2! as is indicated by the inclusion of still another direct source 46. In practice the several D. C. potentials would generally be derived from rectified and filtered alternating current.

In Figs. 9 and 10 are illustrated a combination which includes a deflecting magnet 41 between the pole pieces of which the X-ray tube I is mounted. The deflecting magnet 41 takes the place of one of the two pairs of electrostatic deflcctor plates. It may be substituted for either pair. If substituted for plates 24 and 25 the winding of magnet 41 would be supplied with sixty cycle current; but if the magnet is used as a substitute for deflector plates 26 and 21 its winding would be supplied with low frequency current.

As previously stated, the cathode beam is deflected transversely of the magnetic lines of force passing between the pole pieces. 0n the contrary, the cathode beam is not deflected transversely of the electrostatic lines of force between the deflector plates but, instead, it is deflected toward the positive plate. This accounts for the space relationship between the deflector plates 28, 21 and the pole pieces of magnet 41 as illustrated in Fig. 10.

It is possibleto use two sets of deflecting magnets and thus eliminate both sets of electrostatic deflector plates, but it is thought that it would be necessary to provide considerable separation between the two magnets in order to keep their space fields sumciently isolated.

Figs. 11 and 12 illustrate the construction of the cathode end portion of a tube similar to that of Figs. 4, 5 and 6, except that a cathode beam of linear form (rectangular cross-section) is projected. In this case the instead of being in the form of a conical spiral is of linear form; the aperture 49 in the focusing shield and likewise the passageway 50 through the gun 23 are rectangular in form. Only one set of deflector plates 26 and 21 is required because the linear beam scans the target only in one plane, i. e. laterally of the linear focal-spot. A deflecting magnet may be used with the tube structure of Figs. 11 and 12 to take the place of the electrostatic deflector plates 28 and 21.

What is claimed is:

1. The combination in an X-ray tube comprising a target, a cathode and focusing means, of means for cyclically deflecting the cathode beam thereby causing the focal-spot cyclically to scan the target, and means for cyclically moving the target isochronously with the cyclical scanning movement of the focal-spot whereby to maintain the focal-spot substantially stationary in space.

2. The combination in an X-ray tube comprising a target, a cathode and focusing means, of means for cyclically deflecting the cathode beam thereby causing the focal-spot cyclically to scan the target, and means for cyclically reciprocating the target isochronously with the cyclical scanning movement of the focal-spot whereby to maintain the focal-spot substantially stationary in space.

3. The combination in an X-ray tube comprising a target, a cathode and focusing means, of electrical means for cyc'lically deflecting the cathode beam thereby causing the focal-spot cyclically to scan the target, and means for cyclically reciprocating the tube together with said target isochronously with the cyclical scanning movement of the focal-spot whereby to maintain the focal-spot substantially stationary in space.

4. In combination, an X-ray tube comprising an evacuated envelope, a target and a cathode within said envelope, focusing means for concentrating electrons propagated by said cathode into a beam and directing said beam toward said target, two sets of electrical deflecting means operable individually to deflect said beam laterally in different directions respectively, one of said sets of electrical deflecting means being connected to a source of relatively high frequency alternating potential, the other of said sets of electrical deflecting means being connected to a source of relatively low frequency alternating potential whereby the focal-spot projected upon said target is caused to scan said target cyclically in two directions substantially at right angles and means for reciprocating said tube cyclically in the same plane as that in which the focal-spot scans the target at low frequency thermionic cathode 4l the reciprocatory movement of said tube being isochronous with the low frequency scanning movement of the focalt.

5. The combination with an X-ray tube comprising a target and means for projecting a 'cath-' flecting the cathode beam whereby to cause the focal-spot to traverse the face of the target, and mechanism for moving the x-ray tube bodily in unison with the aforementioned traversing movement of the focal-spot but oppositely in direction whereby to maintain the focal-spot substantially fixed relatively to the subject being radiographed, said last-mentioned means and said mechanism being so interassociated as to operate in timed relation one to the other.

7. The combination with, an X-ray tube comprising an envelope, a target, a cathode and focusing-means within said envelope, of means for to maintain the focal-spot substantially fixed relatively to the subject being radiographed, said 8. The improved method of operating a radio-,

graphic X-ray tube of thetype comprising a. target, a cathode and cathode-beam focusing means within an envelope and wherein the cathode beam is caused to impinge upon the face of the target and produce thereon a focal-spot, said improved method comprising: moving the cathode beam laterally of its direction of prop tion so as to cause the focal-spot to traverse the face of the target and, at the same time, moving the X-ray tube bodily in a direction opposite to the lateral movement of the cathode beam at substantially the same speed as that at which the focal-spot traverses the face of the target so that the focalspot remains substantially stationary relatively to the subject being radlographed notwithstanding its movement relatively to the face of the target.

9. The improved method of operating a radiographic X-ray tube of the type comprising a target, a cathode and cathode-beam focusing means within an envelope and wherein the cathode beam is caused to impinge upon the face of the target and produce thereon a focal-spot, said improved method comprising: oscillating the cathode beam laterally of its direction of propagation so as to cause the focal-spot oscillatorily to traverse the face of the target and, at the same time, oscillating the X-ray tube bodily so that the face of the target moves always in directional opposition to the movement of the focal-spot thereacross, the oscillatory movement of the face of the target being maintained always at substantially the same speed as that at which the focal-spot traverses the face of the target, the oscillatory moveshield within said a chamber which ments of both the x-ray tube and the focal-spot occuring in the same plane, said plane of oscillation being such that the focal-spot is maintained substantially stationary relatively to the subject being radiographed.

10. The combination in an x-ray tube of -an evacuated envelope, an electronic-emissive cathode and an anode in spaced relation within said envelope, means for concentrating the electronic emission from said cathode into a beam and directing said beam toward said anode, a shield forming a space within said envelope which is isolated to a substantial degree from the influence of said anode so that the electrons emitted by said cathode are not greatly accelerated by the anode field,- at least until they have traversed a substantial portion of the space path between' said cathode and said anode, said cathode being situated within said isolated space, said shield having an opening to permit passage therethrough of the cathode beam, and electrical means operative upon said beam within said isolated space for deflecting said beam laterally of its direction of propagation.

11. The combination inan X-ray tube of an evacuated envelope, an electronic-emissive cathode and an anode in spaced relation within said envelope, means for concentrating the electronic emission from said cathode into a beam and directing said beam toward said anode, a tubular envelope, said shield forming is largely isolated, by virtue of the shield, from the influence of the anode electrostatic field, said cathode being situated in said chamber, said shield having an opening for the passage therethrough of the cathode beam, means for impressing a positive high potential on said lower than that of the anode, upon said shield, and electrical means operative within said chamber to deflect said beam laterally of its direction of propagation.

12. The combination in an X-ray tube of an evacuated envelope, an electronic-emissive cathode and an anode in spaced relation within said envelope, means for concentrating the electronic that of the anode, upon said shield, and means for impressing potentials of opposing polarities upon said deflecting electrodes.

'13. The combination in an X-ray tube of an evacuated envelope, an electronic-emissive cathode and an anode in spaced relation within said envelope, means for concentrating the electronic emission from said cathode into a beam and directing said beam toward said anode, shielding means dividing the space path between said cathode and anode into at least two successive parts,

at least one of said parts being electrically isolated, at least to a substantial degree, from the influence oi said anode, said shielding means having an opening to permit passage of said beam to said anode, the arrangement being such that the electrons constituting said beam move toward said anode through an initial portion of the space path at a relatively low velocity and, thereafter, at a relatively high velocity, electrical beam deflecting means adiacent a portion of the space path wherein the electrons move at relatively low velocity, means for impressing a relatively high positive potential on said anode with respect to said cathode, and means for impressing a lower positive potential on said shielding means with respect to said cathode.

l4. Thecombination in an x-ray tube of an evacuated envelope, an electronic-emissive cathode and an anode in spaced relation within said envelope, a gun adjacent said cathode, an elongated tubular shield enveloping said cathode and gun, and at least one pair of electrostatic deflecting electrodes within said tubular shield, said electrodes being arranged in mutual opposition and operative to effect lateral deflection of the beam of electrons propagated from said cathode, means for impressing a positive high potential on said anode, means ior impressing a positive potential, lower than that of the anode, upon said shield, means for impressing a potential upon said gun and means for impressing a varying potential across said deflecting electrodes.

15. The combination in an electrical discharge device of an evacuated envelope, an ion-emitting electrode and a second electrode in spaced relation within said envelope,'means for concentrating the ionic emission from said first-mentioned electrode into a beam and directing said beam toward said second electrode, a shield forming a space within said envelope which is isolated to a substantial degree from the influence of said second electrode so that the ions emitted by said first electrode are not greatly accelerated by the ileld or said second electrode, at least until they have traversed a substantial portion oi the space path between said electrodes, said first electrode being situated within said isolated space, said shield having an opening to permit passagetherethrough of said beam, and electrical means operative upon said beam within said isolated space for deflecting said beam laterally of its direction oi propagation.

, CHARLES V. LI'I'I'ON. 

